Implantable medical devices include, among other things, cardiac rhythm management (CRM) devices such as pacers, cardioverters, defibrillators, cardiac resynchronization therapy (CRT) devices, as well as combination devices that provide more than one of these therapy modalities to a subject. Such implantable devices are typically powered by a battery. When the battery's useful life has been exhausted, the implanted device is typically explanted and replaced before the cessation of therapy. Therefore, it is often useful to know how much battery capacity has been used and/or how much battery capacity remains. Some battery gauges display remaining battery capacity in amp/hours (Ahr, units of charge remaining). While useful in some respects, a display in Ahr can be confusing to a clinician or laypersons who are not accustomed to working with units of charge. For clinicians, battery longevity in units of time would be preferable because patient follow-ups are scheduled on intervals related to time, i.e. 3 or 6 months. Improved systems and methods for battery management and battery longevity displays in implantable medical devices are needed.